The original name and definition were though to describe only galaxy groups. However, many studies [1]}, [2]}, [3]} found the existence of fossil clusters. This is due to the lack of an upper limit for the X-ray luminosity in both the most-common operational definitions. As a consequence, when describing the general properties of the population, fossil groups and fossil clusters are interchangeable in the literature. Besides, a more general fossil systems is also widely used. We invite the reader to consider these three definitions as equivalent along this review.
